Electrochemical detection cell

ABSTRACT

An electrochemical cell comprising an anode, a cathode and a reference electrode operating in an aqueous electrolyte is utilized for detection of noxious gases in air. The gas is oxidized at the anode and detection thereof occurs as a result of the current generated by the reaction. A fixed potential difference is maintained between the anode and the reference electrode to avoid generation of undesired current from reactions involving an oxygen-water redox couple within the cell which would invalidate anode-cathode current for gas detection purposes. The fixed potential is chosen from within the range of about 0.9 to 1.5 volts.

BACKGROUND OF THE INVENTION Field of the invention

The present invention relates to electrochemical cells, and particularlyto the structure and arrangement of a cell especially suitable fordetection and measurement of noxious gases in the atmosphere.

Discussion of the prior art

In recent times, great awareness has developed regarding the dangers ofair pollution, particularly in urban or industrialized areas. As thelevel of noxious elements in the atmosphere increases, a greater needarises for equipment to detect and measure the quantity of such elementsso that their presence in the atmospheres can be reduced or eliminated.In order to meet needs arising in connection with pollution control,extensive activity has been devoted to development and production ofequipment useful in solving this problem. For the successful developmentof such equipment, primary consideration must be accorded to therequirements of commercial and operational feasibility. Although systemsmay exist which may be considered functionally successful, actualutilization in practical applications has quite often been thwarted dueto the cost or complexity of such equipment. Therefore, in many caseswhere beneficial reduction of air pollution has been an importantdesideratum, its achievement has been rendered impractical by theinordinately complex or costly aspects of the means proposed therefor.

Accordingly, there exists an urgent present need for air pollutioncontrol equipment which is both effective in operation and which can bepractically utilized in widespread commercial applications withoutincurrence of excessive cost. This requirement exists in connection withequipment for the detection and measurement for polluting materials, aswell as for equipment whereby the quantities of such materials may becontrolled or reduced.

The general criteria applied to measuring and testing equipment such asthe cell of the present invention include requisites for portability,non-prohibitive cost and accuracy in measuring the quantity of the gasdetected. In the prior art, it has been found difficult tosimultaneously fulfill all of these requirements. Increasing theaccuracy of measuring equipment has inherently involved an increase ineither the size or the complexity of such equipment therebydisadvantageously affecting either cost or portability or both. Quiteoften, problems related to the simultaneous provision of these featureshave been decisive in obstructing the practical development andutilization of particular detection apparatus.

It is, therefore, considered of significant importance and a valuablecontribution to the art of pollution control equipment to providedetection apparatus capable of accurately measuring gas quantity whichis also of a relatively convenient size enabling portability, and whichdoes not involve prohibitive cost for its manufacture and practicalutilization.

SUMMARY OF THE INVENTION

Briefly, the present invention may be described as an electrochemicalcell for the detection of noxious gases, said cell comprising an anode,a cathode, an aqueous electrolyte, means for exposing the anode to asubstance to be detected, means defining a reference potential, andmeans for maintaining a fixed potential upon said anode relative to saidreference potential, said fixed relative potential being from within arange wherein an oxygen-water redox couple within the cell isineffective to generate current at a level which is discernible relativeto the level of current produced therein by a reaction involving thesubstance to be detected.

By a more specific aspect of the invention, the fixed relative potentialis chosen from within the range of about 0.9 to 1.5 volts anodicrelative to the hydrogen couple as a zero base.

By another specific aspect of the invention, the cell is constructed tocomprise an anode chamber defining a labyrinthine path through which theair is pased to appropriately expose to the anode the substance to bedetected. Alternatively, the anode chamber may comprise propeller meansfor effecting such appropriate exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention may be had by referenceto the following detailed description of the preferred embodimentsthereof taken in connection with the accompanying drawings wherein:

FIG. 1 is a view in perspective of a cell embodying the principles ofthe present invention;

FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1;

FIG. 3 is a side elevation partially broken away of an interior portionof the cell of FIG. 1 depicting in better detail the anode chamber ofthe cell;

FIG. 4 is a side elevation partially broken away showing the cathodestructure of the cell which, in FIG. 1, is out of view on the back sidethereof;

FIG. 5 is a partial view in perspective of an alternative embodiment forthe anode chamber of the cell of the invention;

FIG. 6 is a chart derived from the Electromotive Series of Elementsindicating for exemplary redox couples theoretical relative electrodepotentials determining whether a couple will undergo an oxidation or areduction reaction;

FIG. 7 is a curve depicting the nature of the relationship betweencurrent which may be developed in a cell due to an oxygen-water redoxcouple and applied electrode potential; and

FIG. 8 is a schematic diagram of a potentiostat circuit for controllingoperation of the cell and particularly as applied in maintaining a fixedrelative potential difference between the cell anode and a referenceelectrode.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now in detail to the drawings, there is shown in FIG. 1 anelectrochemical detection cell embodying the principles of the presentinvention which comprises an electrolyte container 10 having a liquidelectrolyte 12 therein with an electrolyte matrix 14 extending fromwithin the electrolyte to between the electrodes of the cell. The matrix14 is formed from fibrous glass material and operates as a wick havingthe electrolyte absorbed therein in a liquid phase resulting from itscontinued immersion in the reservoir 12.

A pair of support members 16 and 18 mounted on opposite sides of theelectrolyte matrix 14 at the upper end thereof operate to retain,respectively, an anode 20 and a cathode 22 in contact with theelectrolyte matrix 14. The anode 20 may be mounted or embedded on thesupport member 16 in any known manner, with the cathode 22 beingsimilarly mounted upon the support 18. The supports 16 and 18 operate toprovide structural mounting for the electrodes 20 and 22 and to maintainthe electrodes in operable electrochemical relationship with regard tothe electrolyte contained within the matrix 14.

A third or reference electrode 24 is also mounted upon the supportmember 16, in the same manner as the anode 20 but spaced slightlytherebelow, in contact with the electrolyte matrix 14.

The support member 16 is structured to define a labyrinthine path on theinterior side thereof upon which the anode 20 is supported. Thelabyrinthine path is generally designated by the numeral 36 and isdefined within a generally rectangular cavity formed by inner walls 28,30, 32 and 34. A plurality of horizontal spacer members 38, 40 and 42extend from the sidewalls 30 and 34 in alternating fashion to verticallyspace the channels of the labyrinthine path 36. An inlet conduit 44extends through the support member 16 to establish gas flowcommunication into the labyrinthine path 36 and an outlet conduit 46permits exit gas to flow out of the labyrinthine path 36.

At the lower end of the support member 16, there is provided a generallyrectangular opening 48 defined by inner walls 48a, 48b, 48c and 48dextending completely through the support member 16 to permit thereference electrode 24 to be exposed to the surrounding atmosphere.Similarly, and as best shown in FIGS. 2 and 4, the support member 18 hasdefined therethrough a generally rectangular opening 50 bounded by innerwalls 50a, 50b, 50c and 50d, which permits exposure of the cathode 22 tothe ambient atmosphere.

In the operation of the electrochemical cell of the present invention,atmospheric air containing a noxious impurity such as carbon monoxidewhich is to be detected is introduced at a metered rate into the anodechamber defined by the labyrinthine path 36 through the inlet conduit44. As the air is passed over the anode 20, the electrochemical reactionwhich occurs by virtue of the exposure of the impurity to the anode 20will produce a current in the external circuit of the cell therebyenabling detection and measurement of the impurity. Passage through thelabyrinthine chamber 36 of air samples containing an impurity to bedetected will effect adequate exposure to the anode 20 of the impurityin a manner providing an appropriate reaction rate of the impurity as aresult of the fact that the air will be exposed over a relativelyconstant area of the anode 20.

The requirement that the impurity to be detected be exposed to the anodeover a relatively constant anode surface area relates to the obviousnecessity for insuring that any change which takes place in themeasurement reading of the output circuit occurs as the result ofchanges in impurity concentration, and not as a result of phenomenonwhich bears no relationship to the measurements desired. If the changeoccurs as a result of other factors, not related to or indicative ofimpurity concentration, the cell will be rendered less accurate orinoperative. Accordingly, it will be understood that exposure of theimpurity of the anode in a sporadic or uncontrolled manner which wouldunpredictably vary the level of anode area exposed, will effectvariations in the impurity reaction occurring at the anode therebyadversely affecting the validity of the output reading.

It will also be appreciated that variation in the rate of flow of theair sample through the anode chamber defined by the labyrinthine path 36can also produce undesired variations in the output reading. Therefore,in the specific embodiment depicted in FIGS. 1-4, the air sample ispassed through the labyrinthine path 36 at a metered rate to insure thatchanges in the output reading are the result of changes in impurityconcentration and not caused by unpredictable flow rate variations.

Accordingly, it will be understood that, as a general rule, the cell ofthe present invention must be operated in a manner to insure thatchanges in the output reading are a valid and accurate representation ofchanges in the concentration of the impurity to be measured. Any otherfactors, such as exposed anode area or air sample flow rate, must eitherbe maintained constant or controlled and counterbalanced in order tonegate the net effect thereof, in a manner which will be apparent tothose skilled in the art, to insure valid operation of the cell of thepresent invention.

After having been scrubbed by reaction at the anode 20 of the detectedimpurity, the air sample is passed out of the anode chamber through theoutlet conduit 46.

The basic principles underlying the present invention may be morereadily explained by reference in the description of a preferredembodiment to operation of the cell in connection with detection of aparticular substance or impurity. Although the principles of theinvention may be utilized in cells appropriate for detecting any one ofa variety of noxious substances, for the purposes of facilitating thepresent description it will be assumed that the cell depicted in FIGS.1-4 is constructed to react and detect carbon monoxide from an airsample passed through the labyrinthine path 36.

Accordingly, in the cell of FIGS. 1-4 arranged for detection of CO, theelectrolyte 12 is an aqueous solution of sulfuric acid and is maintainedat ambient room temperature. The material chosen for the anode 20 isplatinum black, and the electrode shown is a Teflon-bonded diffusionelectrode, well known to those skilled in the art both as to compositionof material and physical structure. Although a variety of othermaterials may be chosen from within the knowledge of those skilled inthe art for the cathode 22 and the reference electrode 24, in thepreferred embodiment of FIGS. 1-4, these electrodes are also selected tobe Teflon-bonded platinum black diffusion electrodes similar to theanode 20.

An important requirement of the present invention is the maintenance ofa fixed relative potential between the anode 20 and the referenceelectrode 24. The circuitry whereby this is accomplished is shown inFIG. 8, and a more detailed description of the arrangement and operationthereof will be provided hereinafter. FIG. 8 depicts a potentiostatcircuit which is generally arranged in accordance with conventionalprinciples within the knowledge of those skilled in the art and whichenables the maintenance of the fixed relative potential between theanode 20 and the reference electrode 24 without development of currentflow therebetween. The circuit also operates to enable appropriatecurrent flow in the external circuit between the anode 20 and thecathode 22 when an impurity such as carbon monoxide is reacted withinthe cell of the invention.

The significance of the fixed relative potential which is maintainedbetween the anode 20 and the reference electrode 24 may be bestunderstood from the chart of FIG. 6 which depicts the relationshipbetween theoretical reversible electrode potential and its effect uponreactions which will take place when certain elements are exposed withina cell to an electrode with an applied potential. The chart of FIG. 6 isderived from the Electromotive Series of Elements, and sets forth by wayof example certain electrochemical couples and their theoreticalreversible electrode potentials. The electrochemical redox couples areset forth on the right side of a vertical scale with related theoreticalelectrode potentials indicated therealong on the left side.

The value of the theoretical reversible electrode potential of a redoxcouple will determine whether the couple will undergo oxidation orreduction as a result of exposure to an electrode having a potentialapplied thereto. Thus, if a potential is applied to an electrode suchthat it is more anodic than the reversible electrode potential of thecouple, the reduced species of this couple will be oxidized. Conversely,if a potential is applied to an electrode such that it is more cathodicthan the reversible electrode potential of the couple, then the oxidizedspecies of this couple will be reduced.

As previously stated, the preferred embodiment of the present inventiondescribed herein is arranged to accomplish detection of carbon monoxidein an oxygen containing atmosphere. As shown on the scale of FIG. 6, theCO₂ /CO redox couple is indicated as occupying a theoretical level atabout -0.12 volt relative to the theoretical levels of other couples onthe scale. Accordingly, if the cell of FIGS. 1-4 is arranged such thatthe potential difference which is maintained between the anode 20 andthe reference electrode 24 is more anodic than -0.12 to a sufficientdegree, carbon monoxide exposed at the anode 20 will undergo anoxidation reaction and the CO₂ /CO couple will go in the directionindicated by the following formula:

    CO+H.sub.2 O→CO.sub.2 +2H.sup.+ +2e.sup.-

Of course, it will be understood that the degree to which an actuallyapplied potential level must be more anodic or more cathodic than thetheoretical reversible electrode potentials will vary depending uponspecific circumstances. However, it should be understood that thespecific level differences required under practical circumstances willbe within the knowledge of those skilled in the art.

One of the problems which may be encountered in the utilization ofmeasuring equipment such as the cell of the present invention relates tothe fact that more than one impurity may be exposed to the workingelectrode of the cell. Accordingly, the reactions which occur at theworking electrode would produce current indicative of more than oneimpurity and it would be impossible to distinguish the presence andquantity of a particular impurity. In most practical applicationsrelating to atmospheric air, the level of carbon monoxide in the air farexceeds the level of other impurities such as nitric oxide andhydrocarbons. Accordingly, in most ordinary situations involvingatmospheric air it will not be necessary to make provisions todistinguish between the elements in the air since any readings which aregenerated in the output circuit will be predominantly the result ofcarbon monoxide presence. Of course, if a very high degree of accuracyis required, or if the level of nitric oxide or hydrocarbons issufficiently high to invalidate the accuracy of the cell output,provisions must be made to enable the various impurities to bedistinctly detected and measured. A manner for accomplishing this wouldinvolve passing of an air sample through a series of individual cells,with each of the cells being constructed to react one only of aplurality of impurities with the cell environment being inert to theother impurities. This can be accomplished by appropriate selection ofanode and/or cathode materials, the substance for the electrolyte, andthe temperature at which the electrolyte is maintained.

Thus, in a situation involving atmospheric air, a series of three cellsmay be individually structured and appropriately arranged to have an airsample passed therethrough in series in a manner whereby each cell willdetect a single impurity. The sequential arrangement of the cells willbe important since in some cases a preceding cell will scrub or reactall of the impurity contained in an air sample thereby making thatimpurity unavailable for reaction in a subsequent cell. A more detaileddescription of an arrangement whereby the individual impuritiescontained in atmospheric air may be separately detected will be providedhereinafter.

Another more significant problem with regard to generation of undesiredunauthentic current in the external circuit, and the problem with whichthe present invention is primarily concerned, relates to the fact thatan oxygen-water redox couple will be potentially available within thecell to produce, in the external circuit, current which is not derivedfrom reaction of the impurity to the detected. Such a redox coupleresults from oxygen contained in the incoming atmospheric air and watercontained in the electrolyte. For example, under certain circumstanceswater may become oxidized at one of the electrodes of the cell therebygenerating current in the external circuit that would not bedistinguishable from the current generated by the impurity reaction.Likewise, oxygen may undergo reduction within the cell thereby similarlygenerating undesired current.

In accordance with a basic principle of the present invention, anelectrochemical cell may be arranged to effectively abate current whichmight result from reactions of an oxygen-water couple within the cell.

The mechanism of the present invention enabling control within the cellof the oxygen-water couple is the maintenance of a fixed potential uponthe anode relative to the reference electrode creating a conditionwhereby the oxygen-water couple produces in the external circuit nodiscernible current relative to the current produced by reaction of theimpurity. In accordance with the principles of the invention, it hasbeen found that a fixed potential selected from within the range between0.9 and 1.5 volts relative to the reversible electrode potential of thehydrogen couple will enable achievement of the benefits of theinvention.

Referring now to FIG. 6 in order to more clearly understand the specificselection of preferred potential ranges, it will be seen that theoxygen-water redox couple is referenced upon the scale at +1.23 volts.This indicates that at an electrode having a potential more cathodicthan +1.23 volts there would occur a reaction involving reduction ofoxygen. If the potential of the electrode was chosen to be more anodicthan +1.23 volts then oxidation of water would occur at the electrode.Of course, with an electrode potential established in the region between0.9 and 1.50 volts, any couple in a region more cathodic thereto wouldundergo oxidation. For example, with a potential in this region,oxidation of CO would invariably occur due to the fact that the CO₂ /COcouple is referenced on the scale at -0.12 volt, which is a levelsignificantly more cathodic than the level at which the electrodepotential would be established.

The curve of FIG. 7 is intended to depict the general relationship whichexists between current developed in a cell due to an oxygen-water redoxcouple and the level of potential applied to the electrode at which thereactions occur. It will be noted that in curve A, at a level of +1.23volts no current will be generated as a result of reaction at anelectrode charged at that level. As the level of charge upon theelectrode is varied, anodic or cathodic current will commence to begenerated depending upon the direction in which the potential level isvaried. However, it is important to note that in order for there to begenerated current of any consequence, it will be necessary that thepotential upon the electrode be at a level which varies to a substantialdegree from the +1.23 volts level.

The curve labeled A generally depicts the situation which would bedeveloped with an electrode comprising platinum. As seen in FIG. 7, asthe potential on such an electrode is varied in a direction either moreanodic or more cathodic than +1.23 volts, little or no current will bedeveloped for a range of potential variation between the points labeledx and y. Only when the potential upon the electrode of curve A becomesmore anodic than the y potential level will there commence to bedeveloped discernible anodic current. Similarly, no discernible cathodiccurrent will be developed until after the potential applied to theelectrode of curve A becomes more cathodic than the x potential level.Thus, it will be understood that if the electrode of curve A has apotential applied thereto which is maintained between the limits x andy, no discernible current will be generated at the electrode as a resultof the oxygen-water redox couple.

The shape and nature of the curve of FIG. 7 will depend primarily uponthe choice of electrode material involved. For different electrodematerials, curves having basically the same shape as curve A may bedeveloped except that the range of applied potentials across which nodiscernible oxygen-water redox couple current will be developed may beacross wider limits. Additionally, the type of electrolyte involved willlikewise affect the specific nature of the curve. For the purpose of thepresent disclosure, it is not deemed necessary to set forth with greataccuracy and detail curves for specific cells since the development ofsuch curves will be within the knowledge of those skilled in the art.However, it is deemed useful to depict the general shape of such curvesso that there may be developed a better understanding of the fact that arange of potential level exists within which no discernible oxygen-waterredox couple current will be generated.

For the curve labeled A, the levels of x and y may be very approximatelyassumed to be 1.0 and 1.7 volts for a platinum electrode. The necessityfor such a high degree of approximation arises due to the fact that muchmore than the material of the electrode must be known in order to moreaccurately establish the value of these levels. Accordingly, the figuresset forth are not deemed of significance other than as a generalindication of the voltage levels which may be involved.

A second example of the types of curves which may be generated is thecurve labeled B, wherein there is depicted the conditions which wouldexist with a gold electrode in an acid electrolyte. Again, veryapproximate levels for the potentials q and r would be, respectively,0.7 and 1.8 volts. Within this range, no discernible oxygen-water redoxcouple current would be generated at an electrode having the indicatedpotential maintained thereupon.

With regard to the selection of electrode material, especially for theworking electrode which in the present case is the anode, the materialchosen should be such that it will operate effectively within the cellto oxidize the particular impurity to be detected. Obviously, oneimportant requirement of the electrode material will be that it isstable in the cell electrolyte. This and other similar conventionalrequirements for an electrode material will be apparent to those skilledin the art. Of more pertinence with regard to the application of a fixedrelative potential is the characteristic of the electrode material toeffect reaction, i.e., oxidation, of the impurity to be detected whenthe electrode is charged at the fixed potential of the presentinvention. It will be found that different electrode materials willproduce differing results to react a given impurity when maintained at aparticular electrode potential. This behavior is especially pertinent inconnection with the current level generated as a result of thereactivity of the impurity at a particular electrode. In some cases, asa result of the particular choice of electrode material and of the levelof potential applied thereto, the reaction of a particular impurity atsuch an electrode may not proceed at a sufficiently high rate togenerate a level of current which will permit an adequate reading in theoutput circuit to detect and measure the impurity reacted. Accordingly,the selection should be made so that for a given impurity an electrodepotential level may be chosen within the limits of the present inventionto produce maximum current for a fixed amount of impurity to be reacted.In this manner, the oxygen-water redox couple current may be eliminatedby virtue of the fixed electrode potential chosen from within the limitsof the invention, i.e., 0.9 to 1.5 volts, with simultaneous enhancementof detection current being provided by virtue of selection of theelectrode material which will operate most effectively to react theimpurity to be detected at the particular fixed electrode potentialutilized.

In the cell of the preferred embodiment of the invention which isdepicted in FIG. 1 and which is intended for the purpose of detectingand measuring carbon monoxide, a range of between 1.07 and 1.13 volts ispreferred for the anode fixed relative potential. The specific preferredfixed relative potential is 1.1 volts. As previously stated, thepreferred material for the anode 20 of this cell is platinum or platinumblack and the electrolyte 12 is chosen to be an aqueous solution ofsulfuric acid. With these parameters, it will be found that for carbonmonoxide there will be generated maximum current when the relativeelectrode potential is maintained fixed at 1.1 volts. As the fixedrelative potential deviates substantially above or below this level thecurrent generated for a fixed quantity of CO reacted will besignificantly reduced. Additionally, at 1.1 volts, any slight deviationin the constancy of this fixed relative potential level will producerelatively less variation in current than would be produced if thepotential were to be maintained at some other level.

Accordingly, it will be seen that although undesired current from anoxygen-water redox couple may be avoided by maintenance of the fixedrelative electrode potential of the present invention, cell accuracy andperformance may be enhanced by appropriate selection of other parametershaving in mind the criteria set forth herein.

Although platinum and platinum black are set forth as preferredmaterials for the CO detection cell of FIGS. 1-4, other materials may besuitably utilized. Other materials which would be suitable forutilization in a cell constructed in accordance with the principles ofthe present invention to detect and measure carbon monoxide may bechosen from the group consisting of platinum, rhodium, iridium,ruthenium, palladium, osmium, tungsten oxide, tungsten carbide,molybdenum oxide, molybdenum sulfide, and alloys or mixtures thereof. Ingeneral and as indicated by the aforementioned grouping, it will befound that anode materials for a CO-detection cell may be appropriatelyselected from the noble metals.

The particular structure and arrangement of a cell formulated within thescope of the present invention may deviate from the specific structureset forth in connection with FIGS. 1-4. The material utilized for thematrix 14 need not necessarily be glass but may be formed to compriseeither silica, zirconia, or various polymers. Additionally, theelectrolyte need not be immobilized by absorption in a matrix but may beprovided in the form of a "free" electrolyte. The importantconsideration in this connection is, however, that the cell be arrangedso that the impurity to be detected and reacted at the working electrodebe permitted to migrate to the interface of the electrolyte and thesurface of working electrode in order to insure reaction of the impuritythereat. Thus, it would be appropriate, for example, to construct thecell of the present invention having a free electrolyte in contact withone side of the working electrode and exposing the opposite side of theworking electrode to the substance, e.g., carbon monoxide, to bedetected. The particular structure of the electrodes set forth in thecell of FIGS. 1-4 comprises a Teflon-bonded porous electrode, and as aresult of the porosity of the electrode the gas to be detected willmigrate from the external surface of the electrode, i.e., the surface ofanode 20 exposed to the labyrinthine path 36, to the internal surfacethereof which is interfaced with the electrolyte, i.e., the oppositesurface of anode 20 which is in abutment with the matrix 14. Replacementof the matrix 14 with a free electrolyte arrangement would not impedethe reactivity of the cell.

The selection of materials for the cathode and for the referenceelectrode of a cell constructed within the scope of the presentinvention may be made within the knowledge of those skilled in the art.Criteria for selection of these materials will relate to commonly knownprinciples of electrochemistry and should be conventionally achievable.For example, the cathode material should, of course, be electronicallyconducting and have low solubility in the electrolyte. Since, as is truein any electrochemical cell of the type described herein, the reactionoccurring at one electrode must be Faradaically equivalent to theopposite redox reaction occurring at the other electrode, it will beunderstood that to complete the electrolytic cell described herein, areduction process must occur at the cathode which will be Faradaicallyequivalent to the oxidation process occurring at the anode. Thus, in anexample of the specific cell described in connection with FIGS. 1-4, thematerial for the cathode should be chosen such that this electrode willbe capable of catalyzing water reduction or oxygen reduction orreduction of the appropriate redox couple having its oxidationcounterpart occurring at the anode, as for example by the oxidation ofCO at the anode 20.

Since the reference electrode is a nonpolarized electrode and does notactively participate in the electrolytic process of the cell, criteriafor selection of materials for this electrode would relate primarily toits ability to cooperate in maintaining the fixed potential relative tothe anode and to its general adaptability to the cell environmentincluding, for example, low solubility in the electrolyte.

As shown in the drawings of FIGS. 1-4, both the reference electrode 24and the cathode 22 are supported in abutment with the electrolyte matrix14 in a manner whereby their opposite sides are exposed to the ambientatmosphere. With regard to the reference electrode 24, an alternativepossibility to this arrangement would be to expose the electrode 24 onlyto air which has been previously passed through the anode chamber andfrom which all or a substantial portion of the carbon monoxide has beenremoved by reaction within the cell. An advantage in using this approachis that it would avoid polarization of the reference electrode 24 whichoccurs as a result of oxidation of carbon monoxide in the ambient air towhich the reference electrode 24 is exposed. Although such polarizationoccurs in the cell of FIGS. 1-4, it is relatively small due to the factthat there is no appreciable gas flow across the surface of thereference electrode 14 and, accordingly, the effects of oxidation of COat this electrode are relatively minor and tolerable. The net effect ofsuch polarization is to cause the anode to become more cathodicresulting in lower readings of CO presence in the air sample passedthrough the anode chamber. Accordingly, the choice of whether to avoidsuch polarization will depend upon the degree of percision required fromthe cell and the practicality of incurring the expenditure involved inthe achievement of such precision.

Another important aspect of the cell of the present invention relates tothe arrangement of the anode chamber through which the substance to bedetected is passed for exposure to the anode surface. As previouslystated, it is important that conditions of the cell irrelevant tochanges in impurity concentration be maintained such that the outputreading of the cell is no invalidated thereby.

As previously stated, the anode chamber arrangement is significant ineffecting appropriate exposure to the surface of the anode of theoxidizable substance to be detected. In FIGS. 2 and 3, the labyrinthinechannel 36 directs the flow path of the CO-bearing air in contact withthe anode 20. The shape and configuration of the channel 36 insures thata substantially constant anode area is exposed to the CO, and assumingan appropriate flow rate, the operation of the cell will be such that nochanges in the output circuit will occur as a result of sporadicvariations in the anode area contacted. Although the labyrinthinechannel 36 is considered an appropriate and preferred approach foreffecting appropriate exposure of the CO within the anode chamber,alternative arrangements are possible within the scope of the presentinvention.

One such alternative arrangement is depicted in FIG. 5 which shows aportion of an electrolytic cell constructed in accordance with thepresent invention and particularly depicting the anode chamber thereof.The cell depicted in FIG. 5 comprises an anode 20a abutting anelectrolyte matrix 14a in an identical manner as depicted in FIGS. 1-4.A support member 60 has the anode 20a mounted therein in a mannersimilar to the mounting of anode 20 upon support member 16. The supportmember 60, instead of providing the labyrinthine channel 36, defines asan alternative thereto an anode chamber 62 which is generally circularin its configuration and which is fully exposed to the surface of theanode 20a. A propeller mechanism 64 driven to rotate by an appropriatemeans (not shown) which will be within the knowledge of those skilled inthe art, operates to swirl air within the anode chamber 62 thereby toenhance the scrubbing effect produced upon the surface of the anode 20a.The inlet means whereby air is introduced into the anode chamber 62comprise a centrally located conduit 66 defined internally of the shaftof propeller 64 in flow communication with the anode chamber 62. An exitconduit 68 extending in flow communication from the sidewall of chamber62 permits the contents of chamber 62 to pass therefrom in a directiontangentially of said sidewall. Thus, air entering the anode chamber 62through the central conduit 66 will be swirled about the chamber by theaction of the propeller 64 thereby effecting oxidation of CO as a resultof contact with the anode surface. Subsequently, the air will be emittedthrough the conduit 68 by virtue of the swirling motion imparted theretoby the propeller 64.

The operation of a cel structured in accordance with FIG. 5 is againeffective to achieve an appropriate distribution across the surface ofthe anode 20a of CO contained in the air introduced into the anodechamber 62. A valid current reading may be obtained in the outputcircuit of the cell of FIG. 5 due to the fact that the CO is uniformlydistributed over a fixed anode surface area. Such fixed uniformdistribution operates in essentially the same manner as the labyrinthinechannel 36 of FIGS. 1-4 to insure that changes in exposed anode surfacearea do not operate to produce erroneous indication of CO concentrationin the output circuit readings. The specific embodiment of FIG. 5 isconsidered especially appropriate for utilization with applicationsinvolving relatively lower CO concentrations since it operates toincrease current levels for a given CO concentration thereby enhancingthe effectiveness of the external circuit readings as indications of thepresence and quantity of CO.

The maintenance of constant potential between the anode and thereference electrode of the cell of the invention is accomplished by apotentiostat circuit, connected to the cell in the manner depicted inFIG. 8, which is conventional and within the knowledge of those skilledin the art. The potentiostat circuit of FIG. 8 operates to maintain aconstant relative potential between the anode and the referenceelectrode.

In FIG. 8, the electrochemical cell of the invention is shownschematically as comprising an anode 70, a cathode 72, and a referenceelectrode 74, with the anode connected through a switch 76 to groundpotential 78. The circuit basically comprises an operational amplifier80 having both the reference electrode 74 and the cathode 72 connectedthereto. A DC power supply 82 having a connection 84 to ground potential78 is connected to the amplifier 80 through leads 86 and 88 withresistors 90, 92, and 94 connected thereacross in parallel between thepower supply 82 and the amplifier 80. Resistor 92 comprises a rheostatand is connected to the amplifier 80 through a lead 96 wherebyadjustment of the resistor 92 enables adjustment of the fixed relativepotential which is to be maintained between the reference electrode 74and the anode 70. The cathode 72 is conected to the amplifier 80 througha resistor 98 having a voltmeter 100 connected thereacross. Thereference electrode 74 is connected to the operational amplifier 80through a lead 102 and as the relative potential between the referenceelectrode 74 and the anode 70 develops a tendency to vary from the fixedlevel established by adjustment of rheostat 92, the amplifier 80operates through a negative feedback to maintain constant the relativepotential between the anode 70 and the reference electrode 74. Thefactor creating the tendency to alter the anode-reference electrodefixed relative potential is developed as a result of reaction at theanode 70 of the impurity to be detected, i.e., oxidation of CO containedwithin the air sample flowing across the face of the anode 70 asindicated by the arrow 104. The output current of the operationalamplifier 80 will pass through the resistor 98 and will be a result ofand related to the level of oxidation of CO occurring at the anode 70.Therefore, the reading taken at the voltmeter 100 will be representativeof the oxidation reaction occurring at the anode 70 and of the quantityof material oxidized. The voltmeter 100 may be readily calibrated in aknown manner to provide determination of the quantity of CO occurring inthe air sample taken, and if the conditions in the anode chamber are inaccordance with the teachings previously set forth, appropriate readingsmay be generated pursuant to the principles of operation provided.

Both the potentiostat circuit of FIG. 8, and the operational amplifier80 included therein, are considered fully conventional and within theknowledge of a skilled artisan.

It should be understood that any deviation which might occur in therelative potential difference between the anode and the referenceelectrode will affect the accuracy and precision of the cell.Accordingly, the extent of deviation which may be tolerated will dependupon the degree of precision required for a particular application. Thepotentiostat circuit of FIG. 8 is considered to provide a degree ofconstancy for the relative electrode potential difference which will beadequate for most applications in connection with atmospheric air. Wherea higher degree of precision may be required circuitry other than thatof FIG. 8, which may be more precisely constructed to insure greateraccuracy, may be used.

Furthermore, it should be appreciated that although the invention isimportantly characterized by the maintenance of a constant or fixedrelative potential difference, deviations in said fixed relativepotential may occur within the concepts of the present invention andwithout departure from the scope and purview thereof.

As previously stated, impurities other than carbon monoxide may bemeasured and detected by cells constructed in accordance with thepresent invention. For example, by providing certain modifications whichmay relate to either the material of the electrode, the electrolytecomposition, or the temperature of the electrolyte, and appropriatelyadjusting the fixed relative potential between the anode and thereference electrode, a cell may be adapted to oxidize a specificimpurity in a manner whereby other impurities simultaneously containedin an air sample will be inert to the cell environment. Inasmuch asnitric oxide and hydrocarbons are the two most significant elements, inaddition to carbon monoxide, which may be usually present in atmosphericair, it is considered appropriate to describe, as examples of cellmodifications, arrangements whereby these elements may be measured,detected and removed from an air sample.

Accordingly, assuming a system wherein it was desired to measure anddetect all three of the more significant impurities present inatmospheric air, i.e., carbon monoxide, nitric oxide and hydrocarbons,this could be accomplished by a three-cell arrangement comprising aseparate cell to individually detect and react each of these impurities.Normally, it would be most appropriate to pass the air sample firstthrough a cell for detection of the nitric oxide. Such a cell shouldpreferably comprise a gold anode and a sulfuric acid electrolytemaintained at room temperature. In this cell, the fixed relativepotential to be maintained between the anode and the reference electrodeshould preferably be from within the range between 1.0 and 1.3 volts. Asa result of passage through the anode chamber of such a cell, the airsample would have removed therefrom all or most of the nitric oxidecontained therein by oxidation at the anode of the cell. The currentdeveloped in the external circuit of the cell as a result of suchoxidation would operate in the same manner as previously described inconnection with FIGS. 1-4 for detection of carbon monoxide, and,accordingly, detection and measuring of the nitric oxide, as well asremoval of all or of a substantial portion thereof from the air sample,could be accomplished.

Subsequent to passage through the nitric oxide detection cell, the airsample would be passed to the cell for detection and oxidation of carbonmonoxide. Such a cell, comprising a platinum electrode and anelectrolyte consisting of sulfuric acid at ambient temperature, may bestructured in accordance with the description previously set forth inconnection with FIGS. 1-4.

A third cell for the detection and measurement of hydrocarbons in theair sample should preferably comprise a platinum black electrode and anelectrolyte consisting of phosphoric acid at a temperature within therange between 100° C. and 200° C. The fixed potential maintained betweenthe anode and the reference electrode should be preferably from withinthe range between 1.05 and 1.15 volts. The air emitted from theCO-detection cell should be introduced into the third cell for detectionof hydrocarbons, with oxidation of the hydrocarbons occurring at theanode in a manner similar to that previously described, to producedexternal current indicating hydrocarbon presence and the amount thereof.

Of course, each of the three cells described should include apotentiostat circuit to maintain the fixed relative potential betweenthe anode and the reference electrode, in the manner previouslydescribed. In each case, undesired current produced by an oxygen-watercouple would be problematic and cold be dealt with and avoided inaccordance with the principles of the present invention from thedescription set forth herein.

As has been stated, because of the specific structure and arrangement ofeach individual cell, no problems will arise in any one of the cellsfrom undesired detection current caused by presence and reaction of animpurity which is not to be detected by that particular cell. Forinstance, in the foregoing arrangement utilizing three cells, the airsamples are first passed through the nitric oxide detection cell. Thepresence in this cell of CO and hydrocarbons will not adversely affectthe validity of the current in the external circuit as a measurement ofNO presence due to the fact that neither carbon monoxide norhydrocarbons will be oxidized in this cell since these elements areinert to the gold anode of the cell. Similarly, the air passed throughthe carbon monoxide cell will not involve oxidation of either nitricoxide or hydrocarbons. Nitric oxide would normally be reactive in the COcell, but since this element has either been removed or reduced toinsignificant amounts as a result of passage through the first NOdetection cell, no problem arises. The hydrocarbons require anelectrolyte other than sulfuric acid at ambient temperature foroxidation to occur and, accordingly, their presence in the CO cell willnot effect a reaction. Therefore, it will be seen that the problem ofplural impurities in an air sample which could obstruct the accuratedetection of a single impurity, is readily dealt with in the mannerdescribed by appropriate selection of cell conditions, i.e., fixedrelative potential and anode and electrolyte characteristics, and byappropriate sequential arrangement of the cells. The problem ofundesired current generated as a result of the oxygen-water couple whichhas also been problematic, will also be readily avoided by theapplication of the appropriate fixed relative potential in accordancewith the principles of the present invention in the manner hereindescribed.

From the foregoing it should be apparent that the principles of thepresent invention will have broad application in cells utilized in avariety of environments for various purposes. Although the foregoingdescription has been limited to the detection and measuring ofimpurities in atmospheric air, it should be understood that theinvention need not be so limited although this will probably be its mostimportant area of application.

Other areas of application for the present invention could be inconnection with industrial equipment, for example, process plants whichrequire detection and measurement of certain gaseous substances. Inconnection with this type of application, it is important to note thatthe substance to be detected may be exposed to the surface of theworking electrode of the cell without oxygen presence. This would notadversely affect the operation of the cell in detecting a particularimpurity or gaseous substance. Since the cell would comprise an aqueouselectrolyte, the impurity exposed at the interface of the electrolyteand the working electrode would be oxidized thereby generating detectioncurrent. It will be clear that exposure to the anode of the impurityalone or of the impurity without oxygen, will not impede occurrence of adetection reaction. Furthermore, removal or absence of oxygen from theimpurity-bearing environment would operate to obviate the necessity forthe lower limit of 0.9 volt in the establishment of the fixed relativepotential between the working electrode and the reference electrode. Itwill be understood that this lower limit is established to insureavoidance of oxygen reduction in the cell which would generate undesiredcurrent. Since in the exemplary industrial application referred to nooxygen may be available to effect this reaction, the problem will notarise and the requirement for the lower limit is removed. However, therequirement for the upper limit of 1.5 volts would remain due to thefact that oxidation of the water in the electrolyte would be apossibility to be avoided. Accordingly, it will be clear that where theimpurity to be detected is not exposed to the working electrode in anoxygen-containing environment, the limits of the present invention maybe defined by a fixed relative potential between the working electrodeand the reference electrode which is not more anodic than +1.50 volts.

Another specific embodiment of the present invention may haveapplication in the detection and measurement of the level of alcohol ina person's breath. Such a cell would be primarily arranged to measureand detect ethanol although methanol would also be detectable with sucha cell. In the specific embodiment of a cell for the detection ofethanol/methanol, sulfuric acid in an aqueous solution would be theprefered electrolyte and the range of fixed relative potential betweenthe anode and the reference electrode would be preferably between 1.05and 1.13 volts.

Although in the foregoing description the present invention has beendescribed by reference to specific preferred embodiments thereof, it isto be understood that modifications and alterations in the structure andarrangement of the invention, other than those set forth herein, may beachieved within the knowledge skilled in the art and that suchmodifications and alterations are to be considered as within the scopeand purview of the invention.

What is claimed is:
 1. An electrochemical cell for quantitativelydetecting a .Iadd.noxious .Iaddend.gas .Iadd.in the presence of air.Iaddend.comprising an anode having catalyst bonded topolytetrafluoroethylene to provide a diffusion electrode, a cathode, areference electrode .Iadd.at which substantially no currentflow.Iaddend., and an aqueous electrolyte .[.in contact with.]..Iadd.within an electrolyte container, each of said anode, cathode andreference electrode having a catalytic surface contacting .Iaddend.said.[.anode, cathode and reference electrode;.]. .Iadd.electrolyte, andsaid reference electrode contacting air, .Iaddend.means for exposingsaid anode to a .[.gaseous substance.]. .Iadd.gas containing a noxiousgas .Iaddend.to be detected; .Iadd.said anode being arranged in saidcell so that one surface of said anode contacts said gas exposing meansand the opposite surface contacts said electrolyte whereby the noxiousgas to be detected migrates from the surface contacting said gasexposing means to the internal surface of said anode to interface withthe electrolyte for reaction in the presence of the catalyst,.Iaddend.means connecting said anode and reference electrode, saidlatter means being a potentiostat .[.for.]. maintaining a fixedpotential on said anode relative to said reference electrode of fromabout 0.9 to 1.5 volts with respect to the potential of the reversiblehydrogen couple in the electrolyte of said cell which potential isindependent of the concentration of the .Iadd.noxious .Iaddend.gas to bedetected and means for measuring said current flowing between sad anodeand cathode of said cell, said measured current being a measure of theconcentration of the gas being detected.
 2. The electrochemical cell ofclaim 1 wherein the anode comprises a material selected from the groupconsisting of platinum, rhodium, iridium, ruthenium, palladium, osmium,tungsten oxide, tungsten carbide, molybdenum oxide, molybdenum sulphide,gold, and alloys or mixtures thereof.
 3. The electrochemical cell ofclaim 2 wherein the anode includes platinum.
 4. The electrochemical cellof claim 2 wherein the anode includes gold.
 5. The electrochemical cellof claim 1 wherein the electrolyte is potassium hydroxide.
 6. Theelectrochemical cell of claim 1 wherein the electrolyte is phosphoricacid.
 7. The electrochemical cell of claim 1 wherein the electrolyte issulfuric acid.
 8. The electrochemical cell of claim 1 wherein theelectrolyte is contained in a matrix.
 9. The electrochemical cell ofclaim 1 wherein the electrolyte is free flowing. .[.10. Anelectrochemical cell for quantitatively detecting a gas comprising ananode, a cathode, a reference electrode, and an aqueous electrolyte incontact with said anode, cathode and reference electrode; means forexposing said anode to a gaseous substance to be detected including achamber behind said anode, said chamber defining a labyrinthine path;means connecting said anode and reference electrode, said latter meansbeing a potentiostat for maintaining a fixed potential on said anoderelative to said reference electrode of from about 0.9 to 1.5 volts withrespect to the potential of the reversible hydrogen couple in theelectrolyte of said cell which potential is independent of theconcentration of the gas to be detected, and means for measuring saidcurrent flowing between said anode and cathode of said cell, saidmeasured current being a measure of the concentration of the gas beingdetected..].